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Session 105 - Galaxies: Counts & Kinematics.
Display session, Saturday, January 10
Exhibit Hall,
The angular momentum in galaxies is routinely ascribed to a process of tidal torques acting during the early stages of collapse. Using second order perturation theory and the Zel'dovich approximation one can predict the angular momentum of a galaxy from the initial mass distribution (Catelan amp; Theuns, 1996, MNRAS, 282:455). These effects should be strongest before the onset of non-linearity and may provide a good framework for study. We have tested this theory for a flat hierarchical cosmogony using a large N-body and hydrodynamic simulation with sufficient dynamic range to both include the tidal fields and to allow resolution of individual galaxies. Galaxies are identified at the present epoch and then traced back to the initial mass distribution. The predictions of theory applied to the initial volume are compared to measurements of the evolved object. Unfortunately, we find poor correlation between the linear theory predictions and the non-linear galaxies. Possible causes for the discrepancy include the relatively slow accretion time for the galaxies during which time the tidal forces do not remain constant. Hierarchical formation also leads to strong jumps in the angular momentum as a function of radius, indicating the dominance of discrete events over a smooth background process. We further find that the mapping between initial and final volumes is not simple and that the total angular momentum of a galaxy is not an empirically derivable quantity. A more predictive analysis of hierarchical models will most likely require the treatment of the fractal nature of structures down to sub-galactic scales.
